Automatic Refresh

Hi, we are having a problem with Zimbra email, we have to click on refresh all the time to check to see if we have new emails.
When I go into Preferences, Mail, Receiving Messages - i'm not getting any options to change the amount of time it takes to refresh the Inbox.
The only options I can choose are: When a message arrives - forward to and when a message arrives from me.
HELP!!! Its driving me mad.
Gemma